XII. Lastly, a person is not obliged by the law of nature to restore money, which has been received upon a dishonest account, or for the performance of a legal act, to which that person was of himself bound. However it is not without reason that some laws have required restitution in such cases. The reason of this is, because no one is bound to part with any thing unless it belongs to another. But here the property is voluntarily transferred by the first owner. The case will be altered, if there be any thing iniquitous in the manner of acquiring the thing; as if, for instance, it be gained by extortion. This gives rise to the obligation of submitting to penalties, which is not immediately to the present purpose. XIII. The present subject may be concluded with a refutation of Medina's false opinion, that a property in things, belonging to another, may be transferred without consent of the owner; provided the things are such as are usually valued by weight, number and measure. Because things of that nature can be repaid in kind, or by an equivalent. But this is only, where such a mode of repayment has been previously agreed upon; or where it is understood to be established by law or custom; or where the thing itself has been consumed, and cannot be identically restored. But without such consent, either expressed or implied, or excepting the impossibility just mentioned, the things themselves must be restored. CHAPTER XI. ON PROMISES. Opinion, that the obligation to fulfil promises is not enacted by the law of nature, refuted--A bare assertion not binding--A promiser bound to fulfil his engagements, though no right to exact the performance of them, is thereby conveyed to another--What kind of promise gives such right--The promiser should possess the right use of reason--Difference between natural and civil law with respect to minors--Promises made under an error, or extorted by fear, how far binding--Promises valid, if in the power of the promiser to perform them--Promise made upon unlawful considerations, whether binding--Manner of confirming the promises made by others, and the conduct of Ambassadors who exceed their instructions, considered--Owners of ships, how far bound by the acts of the masters of such vessels, and merchants by the acts of their factors--Acceptance requisite to give validity to a promise--Promises sometimes revokable--The power of revoking a promise, explained by distinctions--Burdensome conditions annexed to a promise--Means of confirming invalid promises--Natural obligation arising from engagements made for others.